1. Field of the Invention
This invention relates to an injector to introduce a predetermined amount of fluid into or onto a stream of liquid, solid, or gas, and more particularly to a microvalve injector to introduce a fixed volume of fluid into a destination stream.
2. Description of the Prior Art
The need to add a predetermined amount of a fluid into another material is common. Automated methods to add a predetermined amount of fluid to another material are also common. The method of adding a small amount of a fluid to a destination stream is exemplified by the injector apparatus used in a gas chromatograph.
Gas chromatographs are well known for analysis of chemical mixtures, separation of gases and process measurement and control. A gas chromatograph includes three main components: an analytical column which physically separates the components of a sample mixture, a detector to sense the individual components after separation, and an injector to introduce an amount of the sample into the analytical column for separation. For quantitative analysis, the injector must introduce a controlled amount of sample into the analytical column. The most common injection techniques for gas chromatography are by syringe and by sample loop. A fixed-volume of sample is injected into the analytical column using a sample loop injector.
The conventional well known sample loop injector includes a two-position multiport rotary valve. The sample loop is filled with the sample while a carrier gas flows to the analytical column through channels in the valve. When the valve rotor is turned to move the valve into the second position, the arrangement of valve channels is re-configured, and the sample loop becomes part of the channeling that conducts the carrier gas through the valve to the analytical column. In the second position, the carrier gas flows through the sample loop and the sample is swept from the sample loop and flows in the carrier gas stream to the analytical column. If the sample loop is "internal", then the multiport rotary valve has four ports. If the sample loop is "external", then the multiport rotary valve has six ports. Rotary valves are difficult to miniaturize. Relatively large dead volumes and slow switching times limit the performance of rotary sample valves. In addition, rotary valves are limited in the number of cycles before they wear out due to friction at the seal interface.
In the 10S series gas chromatograph manufactured by Photovac International, four solenoid valves isolate the sample loop during filling with sample and insert the sample loop into the carrier gas stream to the analytical column during sample injection.
U.S. Pat. No. 4,474,889, entitled "Miniature Gas Chromatograph Apparatus" issued on Oct. 2, 1984, and incorporated by reference herein, commonly owned with the present application, describes a sample injection scheme which utilizes a miniaturized injector. In this "timed-injection" scheme, the amount of injected sample depends on the period of time that the inject valve is open. However, the amount of sample that flows into the analytical column during a particular injection time decreases undesirably as the sample viscosity increases. Therefore, the reported response (peak area) of the associated gas chromatograph for a particular analyte decreases as the sample viscosity increases.
There is a need therefore for a fixed-volume sample injector without the drawbacks of a rotary valve sample loop, which can be easily miniaturized, and which injects the same volume of sample despite variations in sample viscosity.